The invention relates to a semiconductor optical integrated device, and more particularly to a semiconductor optical integrated device comprising a surface emitting laser element and a photo detecting element, which is usable for parallel optical processing or optical computing applications and parallel optical transmissions.
As high density and high speed parallel optical processing and transmissions are required, the value and importance of improvement in properties of the optical devices used therefor will be on the increase. A possibility of realization of ideal high density and high speed parallel optical processing and transmissions with a great stability of operations thereof depends upon properties of the device such as a high speed performance and a great stability of operations as well as an allowability of a two-dimensional integration of optical device array at a high density and other various factors.
Implementation of the two-dimensional integration of the optical device array requires the optical device to have a structure for surface transmissions of light. The high density integration of the optical device array also requires the optical device to have a heat radiation structure or a structure for removal of unnecessary heat accumulation caused by laser emission and electrical and optical currents flowing through the semiconductor optical device. As the high density of the optical device array is improved, the problem with the heat accumulation in the optical device becomes more serious. Much more improvement in the high density of the optical device array further needs not only the removal of the heat from the optical device but also a possible suppression or reduction of unnecessary electrical and optical currents flowing through the optical device. The reduction of the electrical and optical currents flowing through the optical device may be required by a low threshold current of the surface emitting laser element and great efficiencies of light absorption and light emission of the photo detecting element and the surface emitting laser element respectively. Needless to say, it is important to make a possible minimization of size of the optical device for improvement in the high density of the two-dimensional optical device array.
The low threshold current of the surface emitting laser element permits the optical device to exhibit an improved high speed performance of the operation. The minimization of the size of the optical device also permits a high speed performance of the operation of the optical device.
The stability of the operation of the optical device is essential to secure ideal parallel optical processing and parallel optical transmissions. One of the main factors of instability of the operation of the optical device is in instability of detecting operation of the photo detecting element. Normally, the photo detecting element such as heterojunction photo transistor is also designed to make an absorption of a light having a predetermined wavelength. Actually, a light to be injected into the photo detecting element necessarily has somewhat of variation from the predetermined wavelength. The variation of the wavelength of the light to be injected into the photo detecting element in the optical device causes the instability of the operation of the optical device. Then, such an ideal optical device as having a stability of the operation is sought to include the photo detecting element sensitive to a wide-range of the wavelength of the injection light. It is thus ideal that the photo detecting element is so designed as able to make an absorption of injection lights having a wide-range wavelength but at a possible high absorption efficiency.
Various semiconductor optical devices for emitting and detecting lighs have been developed to be used for the parallel optical processing and the parallel optical transmissions. One of the conventional semiconductor optical device for emitting and detecting lights is disclosed in the Japanese Laid-open Patent Application No. 4-101483 laid open on Apr. 2, 1992, which will be described with reference to FIG. 1.
FIG. 1 is a cross sectional elevation view illustrative of a vertical-to-surface transmission electro-hotonic device with a vertical cavity. The vertical-to-surface transmission electro-hotonic device comprises an n-type GaAs substrate 21, n-type bottom and p-type top distributed Bragg reflectors 22 and 28 and an intermediate layer. The n-type bottom distributed Bragg reflector 22 is formed on the n-type GaAs substrate and comprises 14.5 pairs of alternative laminations of n-GaAs layers 29 and n-AlAs layers 30. The n-GaAs layer and the n-AlAs layers are so designed as to have thicknesses of approximately 672 angstroms and approximately 804 angstroms respectively. The p-type top distributed Bragg reflector 28 is formed on the intermediate layer and comprises 15 pairs of alternative laminations of p-GaAs layers 31 and p-AlAs layers 32. The p-GaAs layer 31 and the p-AlAs layer are so designed as to have thicknesses of approximately 672 angstroms and approximately 804 angstroms respectively. The intermediate layer is sandwiched between the top and bottom distributed Bragg reflectors 28 and 22 serving as reflecting mirrors and comprises bottom and top guide layers and an active layer 25 sandwiched between the guide layers. The bottom guide layer comprises a p-GaAs layer 23 formed on a top surface of the n-type bottom distributed Bragg reflector 22 and an i-GaAs layer 24 formed on the p-GaAs layer 23. The top guide layer comprises an n-GaAs layer 27 formed on a bottom surface of the p-type top distributed Bragg reflector 28 and an i-GaAs layer 26 formed on a bottom surface of the n-GaAs layer 27. The active layer 25 is made of In.sub.0.2 Ga.sub.0.8 As having a smaller energy band gap than that of the i-GaAs layers 24 and 26 sandwiching the active layer 25 so that the top and bottom guide layers may serve as potential barriers to make electrical and optical confinements in the active layer 25. The vertical-to-surface transmission electro-hotonic device with a vertical cavity is biased through top and bottom electrodes 34 and 23 which are provided on a top surface of the p-type top distributed Bragg reflector 28 and on a bottom surface of the n-GaAs substrate 21 respectively, The intermediate layer is so designed as to have a thickness of approximately integer times of a wavelength within medium of laser oscillation. When the cavity length is 9500 angstroms, the thickness of the intermediate layer is approximately 0.3 micrometers.
The vertical-to-surface transmission electro-photonic device with a cavity resonator may be considered as a thyristor. Transmissions of lights or light injection and emission are accomplished though the n-GaAs substrate 21. An injected light through the n-GaAs substrate 21 shows a cavity resonance only when the cavity length defined by the distance between the top and bottom reflective mirrors or the distributed Bragg reflectors 22 and 28 is approximately integer times of a wavelength of the injected light. This results in a stationary wave of the injected light within the intermediate layer. The injected light is then absorbed into the active layer 25 thereby carriers are generated. The generated carriers make the device turn ON where the device is biased. Namely, the device is switched to ON state by the light injection. Then, an injection of electrical current into the active layer 25 appears and causes a population inversion state. In this state, recombinations of electrons and holes appear and therefor spontaneous emissions of lights are generated. The lights of the spontaneous emission are confined and resonated within the intermediate layer between the top and bottom distributed Bragg reflectors 22 and 28 so that an induced emission of a light having a wavelength is caused when an injection current is over the threshold current. The light generated by the induced emission is then transmitted through the n-GaAs substrate 21 as a laser beam.
The above mentioned device, however, has the following problems. The emission light or the laser beam generally has a different wavelength from that of the injection light. As mentioned above, the top and bottom distributed Bragg reflectors 28 and 22 are commonly used as the reflective mirrors for the injection right and the emission light. When the top and bottom distributed Bragg reflectors 28 and 22 as the reflective mirrors are designed to be optimized to the wavelength of the emitting light or the laser beam to obtain a great efficiency of light emission, such reflective mirrors are necessarily mismatched to the wavelength of the injection light. This provides a low efficiency of absorption of the injection light. This also provides undesirable limitation to the absorption wavelength range of the device. It is therefore impossible to design the device highly sensitive to an injection light having a different wavelength from that of the emitting laser beam. This also provides undesirable limitation of area of the light absorption. The above problems are caused by the common structure of the reflective mirrors for the injection light and the emission light, both of which have different wavelengths from each other.
To combat the above serious problems, a discrete integrated optical device comprising a heterojunction photo-transistor (HPT) and a surface emitting laser device (SEL device) had been proposed and disclosed in Electronics Letters 1991, vol. 27, pp. 216-217. With reference to FIGS. 2A and 2B, the device comprises the heterojunction photo-transistor (HPT) and the surface emitting laser device (SEL device). The surface emitting laser device comprises n-type and p-type reflective mirrors and intermediate layer sandwiched between the reflective mirrors. Each of the reflective mirrors comprises plural periods of alternative laminations of AlGaAs layers and AlAs layers. The intermediate layer comprises multiple quantum well active layer of alternative laminations of GaAs layers and AlGaAs layers. The heterojunction phototransistor makes an absorption of the injection light at its base and collector regions and therefor an electrical current is generated. As the phototransistor is biased, the electrical current flows through the surface emitting laser device. The electrical current is injected and absorbed into the multiple quantum well active layer thereby a spontaneous emission of light is caused. The light generated by the spontaneous emission of light is confined and resonated in the active layer between the reflective mirrors so that an induced emission of light or laser beam appears.
The heterojunction photo-transistor serves as a light detecting element, but insensitive to an ideal wide range wavelength of the injection lights. The above conventional heterojunction photo-transistor is required to have a sensitivity to much more wide range of wavelength of the injection light. The heterojunction photo-transistor also provides a poor efficiency of absorption of the injection light.
The light injection and emission are accomplished through the opposite side to the substrate on which the surface emitting laser device and the heterojunction photo-transistor are integrated. This makes impossible to form a heat radiation structure at the device side opposite to the substrate side, for which reason a heat generated by the laser emission or light injection tends to be accumulated in the device portion. As the integration of the optical devices has a high density, the problem with the heat accumulation within the optical device becomes serious. This renders it difficult to improve a high density integration of the two-dimensional array of the optical devices.
Other type of discrete integrated optical device comprising a heterojunction photo-transistor (HPT) and a surface emitting laser device (SEL device ) had been proposed and disclosed in IEEE Photonics Technology Letters, May 1992, vol. 4, No. 5, pp. 479-482. With reference to FIG. 3, the device comprises the heterojunction photo-transistor (HPT) and the surface emitting laser device (SEL device ), both of which are integrated on a substrate in parallel to serve as a NOR logic device. The surface emitting laser device comprises n-type and p-type reflective mirrors and an intermediate layer sandwiched between the reflective mirrors. The heterojunction photo-transistor makes an absorption of the injection light at its base and collector regions and therefor an electrical current is generated. As the phototransistor is biased, the electrical current flows through the surface emitting laser device. The electrical current is injected and absorbed into the active layer thereby a spontaneous emission of light is caused. The light generated by the spontaneous emission of light is confined and resonated in the active layer between the reflective mirrors so that an induced emission of light or laser beam appears.
The above optical device also has the following disadvantages. The heterojunction photo-transistor serves as a light detecting element, Put insensitive to an ideal wide range wavelength of the injection lights. The above conventional heterojunction photo-transistor is required to have a sensitivity to much more wide-range of wavelength of the injection light. The heterojunction photo-transistor also provides a poor efficiency of absorption of the injection light.
The light injection and emission are accomplished through the opposite side to the substrate on which the surface emitting laser device and the heterojunction photo-transistor are integrated. This makes impossible to form a heat radiation structure at the device side opposite to the substrate side, for which reason a heat generated by the laser emission or light injection tends to be accumulated in the device portion. As the integration of the optical devices has a high density, the problem with the heat accumulation within the optical device becomes serious. This renders it difficult to improve a high density integration of the two-dimensional array of the optical devices.
It is therefore required to develop an optical integrated device for emitting and detecting lights which is completely free from the above problems with the conventional device.